Fourteen foals with congenital or neonatal musculoskeletal abnormalities are described. Ten of the fourteen foals had abnormally low total serum T(3) and/or T(4) concentrations suggesting hypothyroidism. Response to thyroid-stimulating hormone was determined in two foals and found to be poor. Seven of the foals were necropsied and six of these had histological evidence of thyroid hyperplasia. These findings suggest hypothyroidism as a cause of congenital musculoskeletal lesions.